This invention relates to a communication system allowing the exchange of information between a central station and several data acquisition and transmission stations via a signal transmission line or cable. More specifically, the system of the invention allows a surface logging apparatus to exchange data with a downhole apparatus which has one or more logging tools that can be lowered into an earth borehole. This exchange may be viewed as a kind of dialog between the surface and the hole. It consists essentially, for the surface apparatus, in controlling the different tools and, for the logging tools, in sending the various logging data to the surface under the control of the surface apparatus.
The invention also relates to a remote telemetry element of the downhole apparatus of an installation for the exploration of earth formations traversed by a borehole. The invention also concerns a logging tool for the exploration of these underground formations. The remote telemetry element and one or more of these logging tools can form a downhole apparatus by connecting them end to end.
Measurements of the characteristics of the different earth formations traversed by a borehole are generally carried out by lowering into the borehole a sonde having a "tool" attached to a cable, with a single or several electric wires, used both for holding the equipment and as an electrical medium for the transfer of data signals from the tool to a reception station on the surface. The maximum rate at which data can be transmitted is limited by the passband of the cable. In practice, this limit is about 80 KHz. In order to reduce downtimes, it is necessary to perform all the measurements as rapidly as possible. To achieve this, a solution has been to simultaneously lower into the borehole several tools in the same combination. In practice, a current procedure is to combine three tools during a single operation. The information from the different tools is sampled and transmitted sequentially with respect to time by means of a multiplexing system. When it is desired to increase the number of tools within a given combination and yet to have the same quantity of data transmitted per tool per unit of time, the sampling rate must be increased. However, as mentioned above, the maximum passband of the cable is limited. It is therefore necessary to strike a compromise between the quantity of data transmitted and the sampling speed. The transfer of the data on the cable must also be achieved with great reliability.
There are known communication systems specially designed to carry to the surface the logging data sent by tools lowered into boreholes. Such a system is described, for example, in the U.S. Pat. No. 3,707,700. It involves essentially the transmission of data coming from nuclear-type tools. These analog data are stored in a central memory located in the tool. The data thus stored are multiplexed, sent to the surface via the transmission cable, and finally demultiplexed on the surface before being recorded.
Another known system is described in U.S. Pat. No. 3,959,767. This system allows the control of motors located in a borehole in accordance with measurements made by apparatus also located in the borehole. The measurements are multiplexed in analog form and then converted into digital values before being processed by a logic circuit. This circuit adds an address to the measurement data to form a complex message which is then sent to the surface through a modulation circuit. The logic circuit also furnishes the address of the motors to be controlled. The controls signals transmitted by the surface apparatus to the downhole apparatus are in analog form.
The U.S. Pat. No. 3,991,611 describes a remote telemetry system designed to send to the surface logging measurements in digital form via a cable at the end of which are attached the logging tools. This system is similar to the digital remote telemetry systems customarily used, for example, in the field of industrial control. The downhole apparatus in this system is comprised of a coding circuit, to which the logging tools are connected in parallel, and a modulator for transmission of encoded data on the cable. The surface apparatus comprises the logging cable and a decoding circuit. The connection of a tool to the coding circuit is specific to the type of tool. Thus, it is not possible to connect a tool of any type whatever without prior adaptation. Moreover, the data from the tools are transmitted to the surface in an order predetermined by a sequence preestablished by a logic circuit. The data from a particular tool cannot be transmitted to the surface in response to a specific command sent from the surface.
Prior art apparatus such as the communication systems just mentioned are directed to the transmission of data in particular cases only; i.e. for well-determined logging tools or for a well-determined combination of tools. These systems do not enable operation in association with other tools, for example, because they deal only with a particular transmission problem. Such apparatus generally allow only one operation mode. Furthermore, these prior art transmission systems generally only transmit data in one direction: from the borehole to the surface. They do not permit a continuous exchange of data between downhole tools and surface equipment. Such an exchange calls for the conveyance of data from the hole toward the surface, and also from the surface downward to the downhole tools.
Whenever several tools are lowered at a time into a borehole, these tools are connected end to end if this is possible. Otherwise, it is necessary to provide adapters for the connections. The electric wires connecting one of the tools to the cable are different from the electric wires of another tool. Thus, the electric wires of a lower tool must go through the tools located over it. This produces a "star" structure. In this case one is quickly limited by the number of tools which may be connected end to end due to the significant space requirements for electric wires. The result is a complete lack of uniformity and operating flexibility.
It is therefore an object of the present invention to provide a well logging communication system which is "universal" with respect to the tools; i.e., which is independent of the type and number of tools used (which can be very high) as well as the order in which the tools are connected.
It is a further object of the present invention to provide a well logging communication system which is reliable, has a high degree of flexibility in operation, and allows the transmission of a much larger quantity of data than has been heretofore possible in prior art systems.